Intraoperative monitoring and screw placement apparatus

ABSTRACT

An intraoperative monitoring module (IMM) for assessing strength of screw attachments to bone and detecting breaches includes a torque sensor on a rotatable tool receptacle and a variable-output current source. The IMM provides concurrent monitoring of rotational torque applied to a screw and stimulus current passing through the screw into bone for evoked electromyography. A motor housing configured to drive in rotation a tool receptacle on the IMM, a screw driver modified for carrying the stimulus current, and a screw attached to the screw driver are optionally included. Cooperative anti-rotation features on the motor housing and IMM support accurate torque measurements and prevent the outer housing of the IMM from rotating with the tool receptacle while a screw is being driven. The motor housing optionally provides electrical power to the IMM.

FIELD OF THE INVENTION

Embodiments are related to surgical devices and more particularly todevices for placing threaded fasteners in bone.

BACKGROUND

Healthy intervertebral discs serve as cushions between vertebrae andallow flexing and bending of the vertebral column. Degenerative discdisease related to the deterioration of intervertebral discs andadjacent vertebrae may lead to pain, impaired range of motion, weakness,numbness, and other undesirable effects. Deterioration of discs orvertebrae may occur as a result of trauma, the natural effects of aging,or other causes. Some examples of degenerative disc disease includeformation of bone spurs and neurological injury from material from aherniated disc intruding into the spinal canal.

Treatment for degenerative disc disease may involve removal of adegenerated disc and/or spurs and fusing the vertebra together. Platesand other devices may be attached to the vertebra with screws to holdthe vertebra in correct position until fusion takes place. Screws arepreferably fixed strongly in bone to provide stable positioning of thebone and avoid damage to the bone and other tissues. The strength ofscrew fixation in bone is important in determining success of surgeryand influences post-operative management. However, bone may be weakenedby illness, age, osteoporosis, and other factors, possibly reducing thestrength of attachment of a screw to the bone. It is therefore desirableto predict the ability of bone to provide a stable, secure attachmentfor a screw along a selected screw insertion path.

The ability of bone to provide stable support for a screw has beenpredicted and assessed by pre-operative and intraoperative methods. Forexample, pre-operative imaging techniques such as bone scans may giveinsight into bone quality for screw placement. Imaging has also beenused intraoperatively to detect bone quality problems relating to screwplacement and advancement, for example bone that is too porous to hold ascrew securely, bone that has cracked, split, fractured, eroded, or hasother structural damage, breaches of a bone surface by a screw, and soon.

Other methods have been applied to assess bone quality for screwplacement. Mechanical properties of bone have been inferred fromelectrical measurements such as electrical resistance of bone, bonethickness and/or strength as detected by distortion of an electric fieldemitted from a probe tip, pressure measurements in screw holes drilledalong the screw insertion path, and others. Pressure measurements,whether by instrument or the surgeon's sense of touch and experience,may give an indication of bone that is too soft, too porous, fractured,or too hard to support a successful screw placement.

The strength of a screw attachment to bone has been estimated fromtorque measurements made during screw advancement through bone. Torquemeasurements may indicate when bone is either too soft or too hard forsuccessful stabilization of a screw advancing through the bone. Attemptshave been made to apply torque measurements to detecting a breach of abone surface by an advancing screw tip. However, torque measurementsalone have been found in some cases to give false indications of abreach having occurred or to have missed breaches later detected byimaging.

A screw breaching a bone surface and coming into contact with a nervemay cause pain, impairment of function of the muscle activated by thenerve, and other unwanted effects. Proximity to a nerve of a screwadvancing through bone has been observed by detecting a response fromthe muscle controlled by the nerve, for example by placing a probe inthe screw hole drilled in the bone and observing an electromyogram (EMG)response by the muscle receiving signals from a nerve close to the screwhole. Stimulus currents injected through a probe placed in the screwhole have been used in conjunction with EMG monitoring, a techniquesometimes referred to as “evoked EMG”. Evoked EMG attempts to determinea threshold magnitude of stimulus current at which a muscle response isdetected. A detected muscle response at a low value of stimulus currentmay correlate to a screw having breached, or close to breaching, asurface of bone close to a nerve. An observed muscle response at ahigher value of stimulus current may indicate the screw is passingthrough structurally sound bone with sufficient bone separating thescrew from adjacent nerves, i.e. no breach has occurred.

In each of the previous methods, screw placement procedures are delayedor interrupted to make assessments of bone quality and security of screwplacement. For example, the screw driver being used to advance the screwinto bone may be capable of limiting and/or measuring applied torque,but the advancement of the screw and the torque measurement are bothinterrupted and the screw disconnected from the screw driver to imagethe insertion path for assessment of bone quality and determine if abreach is imminent or has already occurred. Similarly, previouslyavailable instruments and methods detach the screw from the screw driverbefore determining a value of bone electrical impedance or a thresholdvalue of stimulus current needed to achieve a muscle response for anevoked EMG.

Previously available instruments have not been capable of makingconcurrent measurements of torque and evoked EMG response with a screwdriver that remains in continuous contact with a screw being driven intobone. Switching back and forth between separate mechanical probes,electrical probes, screw drivers, and measurement devices for assessingbone quality for screw placement, whether by imaging, measurement ofelectrical properties of bone and/or muscle response, or measurement ofmechanical parameters such as torque and pressure, delays advancing thescrew to its final stable position, lengthens the amount of time neededto perform a successful screw placement, increases the possibility ofintraoperative and post-surgical complications by the introduction ofadditional instruments at a surgical site on a patient, and raises thecost of performing surgery.

SUMMARY

An example of an apparatus in accord with an embodiment includes a screwplacement tool and an intraoperative monitoring module. Theintraoperative monitoring module includes an outer enclosure and a toolreceptacle rotatably coupled to the outer enclosure. The tool receptacleincludes a tool socket configured for receiving the screw placementtool; a torque sensor affixed to the tool receptacle; and a drivecoupling at an end of the tool receptacle opposite the tool socket. Theintraoperative monitoring module further includes a variable-outputcurrent source; a microcontroller electrically connected to the torquesensor and the variable-output current source; and an optional statusdisplay attached to the outer enclosure. The microcontroller isconfigured to communicate to the status display a value of currentoutput from the variable-output current source and a value of torquedetermined from an output of the torque sensor. The example apparatusfurther includes a motor housing configured for attachment to the drivecoupling and the outer enclosure of the intraoperative monitoringmodule. The motor housing is configured for driving the drive couplingin rotation at a selected rate of rotation and in a selected directionof rotation.

Another example apparatus embodiment includes an intraoperativemonitoring module and a motor housing. The example intraoperativemonitoring module includes an outer enclosure and a tool receptaclerotatably coupled to the outer enclosure. The example tool receptacleincludes a tool socket; a torque sensor affixed to the tool receptacle;and a drive coupling at an end of the tool receptacle opposite the toolsocket. The example intraoperative monitoring module further includes avariable-output current source; a microcontroller electrically connectedto the torque sensor and the current source; and a status displayattached to the outer enclosure. The microcontroller is preferablyconfigured to communicate to the status display a value of currentoutput from the current source and a value of torque from the torquesensor. The motor housing is preferably configured for attachment to thedrive coupling and to the outer enclosure, and for driving the drivecoupling in rotation.

Another example apparatus embodiment includes an intraoperativemonitoring apparatus. The intraoperative monitoring apparatus includesan outer enclosure and a tool receptacle rotatably coupled to the outerenclosure. The example tool receptacle includes a tool socket; a torquesensor affixed to the tool receptacle; and a drive coupling at an end ofthe tool receptacle opposite the tool socket. The example intraoperativemonitoring apparatus further includes a variable-output current sourceelectrically connected to a contact pin on the tool receptacle; amicrocontroller electrically connected to the torque sensor and thevariable-output current source; and a status display attached to theouter enclosure. The microcontroller is preferably configured tocommunicate to the status display a value of current output from thecurrent source and a value of rotational torque determined from anoutput of the torque sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example apparatus embodiment including an intraoperativemonitoring module attached to an optional motor housing and an optionalscrew placement tool.

FIG. 2 shows a view toward a front side of an outer enclosure for theexample intraoperative monitoring module of FIG. 1 .

FIG. 3 shows a view toward a side wall of the outer enclosure for theexample intraoperative monitoring module of FIGS. 1-2 .

FIG. 4 shows a view toward another side wall of the outer enclosure forthe example intraoperative monitoring module of FIGS. 1-3 , furtherillustrating an example of an end of a tool receptacle having a toolsocket for receiving a screw placement tool.

FIG. 5 shows a view toward another side wall of the enclosure for theexample intraoperative monitoring module of FIGS. 1-4 , furtherillustrating an example of an end of the tool receptacle having a drivecoupling configured for connection to a motor housing.

FIG. 6 shows a view toward the side wall of FIG. 3 , illustrating anexample of the tool receptacle with the drive coupling implemented as ahexagonal post.

FIG. 7 shows a cross-sectional view A-A of examples of some componentsincluded in the intraoperative monitoring module of the previousfigures.

FIG. 8 shows a side view of an example tool receptacle included in theintraoperative monitoring module of the previous figures.

FIG. 9 is a schematic diagram of examples of components and electricalconnections included in the intraoperative monitoring module of theprevious figures.

FIG. 10 shows a view toward a side of the example motor housing fromFIG. 1 .

FIG. 11 shows a view toward a rotatable chuck on the example motorhousing from FIG. 10 .

FIG. 12 illustrates an alternative arrangement of a hand grip and speedcontrol switch for a motor housing in accord with an embodiment.

FIG. 13 shows a partial side view of an example chuck including a screwdriver shank.

FIG. 14 shows a view toward an end of the shank from the example chuckof FIG. 13 .

FIG. 15 shows a side view of an example screw placement tool configuredas a screw driver for tulip head screws used in orthopedic procedures.

FIG. 16 shows a view toward the screw engaging end of the example screwplacement tool of FIG. 1 and FIG. 15 .

FIG. 17 shows a view toward the screw driver shank of the example screwplacement tool of FIG. 1 and FIGS. 15-16 .

DESCRIPTION

An example embodiment of a medical apparatus for secure placement ofthreaded fasteners in bone is configured for concurrent intraoperativemonitoring of torque and generation of stimulus current for evoked EMGwhile maintaining continuous electrical and mechanical connection to afastener being advanced into a bone. The example apparatus includes anintraoperative monitoring module (IMM) configured for mechanical andoptionally for electrical connection to a motor housing and a screwplacement tool. The IMM includes a tool receptacle rotatably coupled toan outer enclosure, a torque sensor for measuring rotational torqueacting on the screw placement tool while a screw is being driven inrotation by a motor in the motor housing, and a variable-output currentsource configured for generating a stimulus current for making evokedEMG measurements. The IMM optionally includes a status display fordisplaying values relating to torque and EMG monitoring and switches forsetting and selecting operational parameters and displayed information.The example medical apparatus optionally includes the screw placementtool. Some apparatus embodiments include the motor housing.

The example medical apparatus is effective for monitoring of torque andstimulus current for evoked EMG concurrently with placement of a screwin bone, without requiring interruption of mechanical and electricalconnections to the screw during the placement procedure. The apparatusis further effective for detecting screw breaches with good confidencein breach detection when used in conjunction with an external EMGsystem, and for assessing the ability of bone along a selected screwplacement path to provide stable and secure attachment of a screw. Thecontinuous monitoring and reporting of parameters relating to thestructural integrity of bone and the proximity of the screw to neuraltissue allows a surgeon to assess bone quality and prospects for a screwbreach or neural interference along a selected screw placement path innear real-time, without interrupting contact between the screw driverand the screw being placed. Apparatus embodiments are therefore wellsuited to continuous monitoring and assessment of bone properties,selection of screw placement locations and screw placement paths, andstrength of screw attachments to bone for pedicle screws used inprocedures on spinal vertebrae. Apparatus embodiments are alsobeneficial for monitoring and assessing screw placements in surgicalprocedures on bones other than spinal vertebrae.

An IMM in accord with an embodiment 100 is optionally provided in asterile package as a single-use item. An IMM configured as a single-useitem is preferably fully functional for the entire duration of asurgical procedure on a single patient, but after the procedure iscompleted the IMM is not re-used and may be disposed of. An IMMconfigured as a single-use item optionally includes a non-rechargeableelectric storage battery and may have less robust protection againsthigh sterilization temperatures and sanitizing solvents than an IMMconfigured for use in more than one surgical session.

An example of an intraoperative monitoring module (IMM) 104 in accordwith a medical apparatus embodiment 100 is shown in FIG. 1 . The IMM 104is configured on a side of an outer enclosure 108 for mechanical andelectrical connection to an optional screw placement tool 102. The IMM104 is configured on a side opposite the screw placement tool forconnection to an optional motor housing 106. A tool receptacle 122rotatably coupled to the outer enclosure 108 is configured on a firstend to receive the screw placement tool and on an opposite end toconnect to the motor housing 106. In an alternative embodiment 100, theouter enclosure 108 of the IMM 104 is an integrally-formed part of themotor housing 106, with the IMM positioned on the motor housing assuggested in the example of FIG. 1 . In another alternative embodiment100, the IMM 104 is an integrally-formed part of the motor housing 106shown in the example of FIG. 12 .

In the example of FIG. 1 , the screw placement tool 102 is configured asa screw driver with a screw engaging end 242 arranged for retaining anddriving a tulip head screw (not shown). Additional features of theexample screw driver for a tulip head screw will be explained later withregard to FIGS. 15, 16, and 17 . Other examples of a screw placementtool 102 include, but are not limited to, a screw driver with an endshaped for driving a Philips head screw, a screw driver with an endshaped for driving an Allen head screw, a screw driver with a socketedend shaped for driving a hex head screw, a test probe for injecting EMGstimulus current into a screw hole formed in bone, a drill for forming ascrew hole in bone, and a tap for threading a drilled hole.

The example motor housing 106 couples to the IMM to drive the toolreceptacle 122 in rotation. The motor housing 106 preferably includes anon/off and speed control switch 224 for controlling a rotation speed ofa motor (not illustrated) inside the motor housing and a motor reversalswitch 226 for controlling a direction of rotation of the motor toselectively advance or withdraw a screw from a drilled hole. The motorhousing 106 preferably receives electrical power for operating the motorfrom an electric power storage battery (not shown) attached inside thehand grip 222 or alternately attached to the outside of the hand grip oranother part of the motor housing.

Depressing the speed control switch 224 on the motor housing preferablyrotates the tool receptacle 122 on the IMM 104 without causing rotationof the outer enclosure 108 of the IMM relative to nonrotating parts ofthe motor housing 106 such as the hand grip 222. Rotation of the toolreceptacle 122 turns a screw placement tool 102 retained in the toolreceptacle at a rotational speed selected by depressing the speedcontrol switch 224. The tool receptacle, a screw placement tool attachedto the tool receptacle, and a screw attached to the screw placement toolrotate together in a direction of rotation selected by the motorreversal switch 226.

Additional features of an IMM 104 in accord with an embodiment 100 areshown in the examples of FIGS. 2-9 . The outer enclosure 108 protectscomponents inside the enclosure from mechanical damage and isolateselectrical components from exposure to water, biological fluids, andcleaning agents commonly encountered during sanitization of the IMM 104and use of the IMM in medical procedures. In some embodiments the outerenclosure is liquid resistant, i.e., the enclosure is configured tooppose liquid entry into interior spaces holding electrical components.

The tool receptacle 122 is rotatably coupled to the enclosure 108 from afirst side wall 112 of the enclosure to a second side wall 114 oppositethe first side wall. The first 112 and second 114 side walls areconnected to one another by a third side wall 116, a fourth side wall118 opposite the third side wall, a front side 110, and a back side 120opposite the front side. In an alternative embodiment, the outerenclosure is configured as a hollow cylinder with end caps, rather thanthe approximately rectangular box shown in the example figures, and thetool receptacle is coupled to the end caps. A tool socket 124 forreceiving the screw placement tool 102 is formed on the tool receptacle122 adjacent the first side wall 112. A drive coupling 125 forconnecting the tool receptacle 122 to the motor housing 106 is formed onthe tool receptacle 122 adjacent the second side wall 114. In theexample tool receptacle 122 of FIG. 5 , the drive coupling 125 isimplemented as a hexagonal socket 126 sized for a sliding fit of ahexagonal drive shaft 233 on some embodiments of the motor housing 106.In the example tool receptacle 122 of FIG. 6 , the drive coupling 125 isimplemented as a hexagonal post 156 extending outward from the toolreceptacle and side wall of the IMM. Some embodiments of the motorhousing 106 include clamp jaws 232 for securely holding the hexagonalpost 156 in a rotatable chuck 230 driven by the motor in the motorhousing.

Anti-rotation features 145 of the IMM 104 and motor housing 106 opposerotation of the outer enclosure 108 relative to the motor housing whenthe tool receptacle 122 is driven in rotation. Examples of anti-rotationfeatures 145 include a first anti-rotation pin 144 and an optionalsecond anti-rotation pin 146 extending outward from the second side wall114 of the outer enclosure 108. The first 144 and second 146anti-rotation pins are positioned to engage with corresponding pinreceptacles 234 formed on a part of the motor housing 106 that does notrotate when the speed control switch 224 is depressed. The pinreceptacles 234 are an example of an anti-rotation feature 145 on themotor housing configured to cooperate with anti-rotation features on theIMM to prevent the IMM from rotating when the tool receptacle is drivenin rotation by motors in the motor housing. The anti-rotation featuresof the IMM 104 hold a status display 128 and other components on thefront side 110 at a constant viewing angle relative to the hand grip andother stationary parts of the motor housing 106. Other examples ofanti-rotation features 145 include, but are not limited to, a bayonetmount with a slotted collar on the outer enclosure positioned to engagea radial pin on the motor housing, a pin extending radially outward fromthe IMM in a direction perpendicular to the axis of rotation of the toolreceptacle engaging a slotted collar on the motor housing, a magnet inthe IMM positioned to couple to a magnet in the motor housing, anaperture formed in the IMM positioned to receive a spring-loadedpushbutton on the motor housing, and a threaded ring on the IMM engaginga threaded collar on the motor housing.

Examples of internal features of an IMM 104 are shown in cross-sectionalview A-A in FIG. 7 . A location and viewing direction for the crosssection is marked by a section line labeled A-A in FIG. 4 . The toolreceptacle 122 is rotatably coupled to the side walls of the outerenclosure 108. The tool receptacle 122 extends from a first side wall112 to a second side wall 114 through the interior of the outerenclosure 108. Flanges 158 on the tool receptacle prevent the toolreceptacle from slipping out of the outer enclosure. Two sealing gaskets160, one near each end of the tool receptacle, oppose liquid intrusioninto the interior of the outer enclosure 108. An O-ring is an example ofa sealing gasket 160 suitable for use in an IMM. A sealing gasket mayoptionally be provided as a circumferential ridge integrally formed onthe tool receptacle. Two rotary bearings 164 attached to at least one ofthe side walls of the outer enclosure 108 support the tool receptacle122 and reduce frictional losses while the tool receptacle is subjectedto rotational torque during placement of screws.

A printed circuit board assembly 168 affixed to a wall of the outerenclosure 108 and/or the status display 128 includes circuits formeasuring and monitoring torque and evoked EMG as will be explained inmore detail with regard to FIG. 9 . In some embodiments 100 the statusdisplay 128 and the printed circuit board assembly 168 are provided asseparate parts connected by a flat cable and in other embodiments 100are mechanically and electrically integrated into one assembly. Whenincluded in an embodiment 100, the status display 128 is connected fordata communication with the microcontroller 208. Examples of a statusdisplay 128 include, but are not limited to, a liquid crystal display, alight-emitting diode (LED) display, and an organic LED (OLED) display.In some embodiments the status display is sufficiently flexible topermit attachment to a substantially nonplanar surface of the enclosure108. Some apparatus embodiments 100 omit the status display 128 from theIMM, instead communicating information to be displayed over a wirelesscommunication transceiver to a remote device such as a smart phone or acomputer monitor. An apparatus embodiment 100 including the statusdisplay 128 may concurrently display information locally on the statusdisplay and remotely on a separate display.

The printed circuit board assembly 128 further includes switches (136,138, 140, 142) accessible from the front side 110. The switches may beprovided as individual switches soldered to the printed circuit boardassembly or may alternatively be provided as a membrane switch assembly143. When included in an embodiment 100, the membrane switch assembly143 optionally includes snap domes between a switch layer and a graphicslayer to provide tactile feedback during switch operation. The membraneswitch assembly 143 optionally includes the indicators (130, 132, 134)attached to a switch layer, with the indicators visible when illuminatedthrough an outer layer of the membrane switch assembly. The membraneswitch assembly is optionally sealed against liquid intrusion.Components and connections on the printed circuit board assembly, forexample a microcontroller, a memory connected for data communicationwith the microcontroller, and other components shown in FIG. 9 , areoptionally protected by a conformal coat to resist damage by exposure towater, humidity, cleaning agents, dust, and biological materials.

Spring contacts 170 establish electrical connections between circuits onthe printed circuit board assembly and circumferential slip rings on anouter surface 198 of the tool receptacle 122. A separate spring contact170 is provided for each of a first slip ring 174, a second slip ring176, a third slip ring 178, a fourth slip ring 180, and a fifth slipring 182. Each spring contact maintains continuous contact with itscorresponding slip ring while the tool receptacle 122 turns through fullrevolutions. Electrical connections between slip rings and a torquesensor are made with electrical conductors (not shown) positioned on theexterior and interior surfaces of the tool receptacle 122 in such amanner that no two of the slip rings are electrically shorted to oneanother.

Electrical contact between the first slip ring 174 and a screw placementtool 102 retained in the tool socket 124 of the tool receptacle 122 isestablished by a spring-loaded contact pin 172 electrically connected tothe slip ring 174 and passing into the void space inside the toolsocket. A strong permanent magnet 184 attached to, or alternately moldedinto, the interior of the tool receptacle holds the screw placement toolfirmly in position in the tool socket 124. For embodiments of a toolreceptacle 122 having a hexagonal socket 126 for receiving a hexagonaldrive shaft 233 from the motor housing 106, the permanent magnet 184inside the tool receptacle, or alternatively a second magnet 184, firmlyholds the IMM 104 against the drive shaft 233.

More details of an example tool receptacle are shown in FIG. 8 . Thetool receptacle 122 may be formed with a hollow cylinder extending fromcircumferential flanges 158 at an end with a tool socket 124 to anopposite end with circumferential flanges 158 and a drive coupling 125.The drive coupling 125 is implemented in some embodiments 100 as ahexagonal socket 126 as shown in the example of FIG. 5 , and in otherembodiments as a hexagonal post 156 as shown in the example of FIG. 6 .A gasket channel 162 formed in the outer surface 198 near each end ofthe tool receptacle is positioned to hold a sealing gasket 160. Similarchannels for sealing gaskets are optionally formed in the first sidewall 112 and second side wall 114 of the outer enclosure 108. The firstslip ring 174, second slip ring 176, third slip ring 178, fourth slipring 180, and fifth slip ring 182 are formed from an electricallyconductive material with low electrical resistance and are attached tothe outer surface 198, extending without interruption all the way aroundthe outer surface.

Torque applied to a tool receptacle 122 retained in the tool socket 124is measured by a torque sensor 260 attached to the tool receptacle. Thetorque sensor 260 includes a strain gauge bridge 202 strongly attachedto the outer surface 198. The strain gauge bridge includes a firststrain gauge 190 and a second strain gauge 192 and optionally includes athird strain gauge 194 and a fourth strain gauge 196. A primarystrain-sensing axis 200 for each strain gauge is preferably oriented atan angle 188 of 45 degrees to the central rotational axis 186 of thetool receptacle 122. In some embodiments 100, all four strain gauges areattached to the outer surface 198 in close proximity to one another. Inother example embodiments 100, the first strain gauge 190 and secondstrain gauge 192 are attached to the outer surface 198 180 degrees awayfrom the third strain gauge 194 and fourth strain gauge 196, i.e., atopposite ends of a diameter of the cylindrical outer surface 198.

Examples of electrical components and electrical connections in an IMM104 included in an apparatus embodiment 100 are shown in an electricalschematic in FIG. 9 . A torque sensor 260 includes an amplifier andfilter circuit 204 receiving electrical signals from a strain gaugebridge 202. A low impedance, low voltage current return connection tothe strain gauge bridge passes through the fifth slip ring 182 and iselectrically connected to other current return points in the IMM 104,for example a common return terminal 150. In some embodiments the commonreturn terminal 150 is one of the anti-rotation pins, for example thefirst anti-rotation pin 144. A bridge excitation voltage Vcc passesthrough the second slip ring 176 connected to an electric storagebattery 166 and optionally to a positive power terminal 148. In someembodiments the positive power terminal 148 is another anti-rotationpin, for example the second anti-rotation pin 146.

The analog signal output 205 of the torque sensor 260, carried on anoutput of the amplifier and filter 204, is electrically connected to aninput of an analog-to-digital converter (ADC). A digitized output 207 ofthe ADC 206 is electrically connected to an input of the microcontroller(MCU) 208. The digitized output 207 corresponds to numerical values ofthe analog output signal 205 from the torque sensor 260. In someembodiments 100 the ADC 206 and the MCU 208 are separate packageddevices and the ADC and MCU communicate with one another over anintervening data and command bus 214. In other embodiments the ADC andMCU are included on one integrated circuit.

The microcontroller 208 is configured to receive digitized strain gaugevalues from the strain gauge bridge 202, calibration values stored in amemory 218, and retrieve from the memory values of other parametersrelating to conversion of strain to torque, and calculate a value oftorque being applied to the screw 300 at the end of the screw placementtool attached to the IMM 104. Calculated values of torque areselectively displayed on the status display 128 and are optionallytransmitted to external equipment through a wireless transceiver 216 indata communication with the MCU 208. Operating commands, networkcommunication addresses for use by the wireless transceiver 216,calculated torque values, minimum and maximum values for preferredtorque limits, stimulus current limits and values for evoked EMGmeasurements, calibration values, sequences of stored torque values foreach fastener being placed, and other data and settings used by the MCU208 are stored in the memory 218. Memory 218 connected for datacommunication with the MCU 208 includes nonvolatile memory 220.

The four strain gauges in the example of FIG. 9 are arranged in a“full-bridge” Wheatstone configuration. Resistors and other componentsmay optionally be added to the strain gauge bridge 202 and/or theamplifier and filter 204 to provide temperature compensation, bridgenulling, and improve noise rejection. In some embodiments 100 the straingauge bridge 202 is arranged as a “half bridge” with only two straingauges.

The IMM 104 further includes a variable-output current source 210configured to output a selected value of stimulus current in response toan electrical signal received over a current setpoint line 212 from theMCU 208. Upon command from the MCU 208, a selected magnitude of stimuluscurrent for making an evoked EMG measurement is output over a continuouselectrical current path 258 with low electrical resistance from thevariable-output current source 210 to the screw 300 on the end of thescrew placement tool 102. The stimulus current flows for a specifiedtime duration from the variable-output current source 210 through thecontinuous electric current path 258 including an output 211 of thevariable-output current source 210, the first slip ring 174, thespring-loaded pin 172 connected to the first slip ring, the screwplacement tool 102, and the screw 300 at the end of the screw placementtool, and from the current path 259 into bone or other tissue in contactwith the screw.

A common return connector 154 attached to the outer enclosure 108 iselectrically connected to the common return terminal 150 for thevariable-output current source 210. An electrode 302, for example a pinelectrode coupled to an electrode connector 306 by an electrode cable304, may be connected to the common return connector 154. The electrode302 may be placed by a surgeon in tissue near a screw insertion path toprovide a reference voltage for output of an accurately determinedmagnitude of stimulus current from the variable-output current source210.

The IMM is capable of outputting the stimulus current concurrently withadvancement of the screw into bone, measurement of corresponding torquevalues, comparison of torque values against stored torque limits, anddisplay of information related to torque values, stimulus currentvalues, and related torque and current limits. Concurrent, as usedherein, is defined with respect to the actions and perceptions of aperson interacting with an apparatus embodiment 100 and refers to eventsoccurring within about one second of one another. An external EMGmonitoring system, not included in the apparatus embodiments describedherein, may detect a muscle response following injection of the stimuluscurrent into bone through the IMM 104, screw placement tool 102, andscrew 300. The surgeon performing the screw placement procedure maycorrelate the detected response from the EMG with values of torqueand/or stimulus current presented on the status display 128 of the IMM104.

When the external EMG monitoring system or a computer in datacommunication with the EMG monitoring system is capable of transmittinga signal corresponding to detection of a muscle response, the MCU 208 isoptionally configured to receive the signal over the wirelesstransceiver 216. An alphanumeric, symbolic, and/or graphical indicationof a muscle response may be displayed on the status display 128 by theMCU 208. The MCU 208 optionally illuminates one or more indicators (130,132, 134) on the front side of the IMM in response to receiving a signalcorresponding to detection of a muscle response. A surgeon may at theirdiscretion continue advancing a screw into bone, determine that a screwis placed in bone with sufficient strength for secure screw retention,reverse the direction of screw rotation to remove the screw, chooseanother location or direction for screw insertion, or take otheractions, according to a combination of torque values and stimuluscurrent determined by and displayed on the IMM at the time of a detectedmuscle response.

Some embodiments of the IMM 104 are configured to display trend data,e.g. a pattern of increasing values or decreasing values over time,optionally relative to specified limits, from historical data stored inthe IMM memory and/or data retrieved from an external storage system.Trend data may be stored, retrieved, and displayed for torque values,magnitude of stimulus current corresponding to a detected a muscleresponse, for the entire duration of a procedure on one patient, foreach individual screw placement, and for other bases for comparison.

The IMM 104 preferably includes several switches and indicators forspecifying data to be displayed on the status display, setting high andlow limits for preferred torque values, setting a magnitude of stimuluscurrent to be output from the IMM, and other controls and monitorsuseful to a surgeon during placement of screws and evaluation of screwattachment to bone. In the example of FIG. 9 , four switches and threeindicators are electrically connected to the MCU 208. Examples of labelsfor the switches and indicators are shown in FIG. 2 . Labels on thefront side optionally use different text than shown in the example ofFIG. 2 .

As an example of operation of an IMM 104, a first switch 136, labelledINC in the example of FIG. 2 , may be activated by a surgeon toincrement or increase a selected value. A second switch 138 labelled DECmay be activated to decrement or decrease a selected value. A thirdswitch 140 labelled SELECT may be activated to select a displayed valuefor a parameter or make some other input choice, or to manually triggeroutput of a selected value of stimulus current from the variable-outputcurrent source 210. A fourth switch 142 labelled MODE may be activatedto step through a set of operational choices.

Information 152 presented on the status display 128 includes text and/orgraphics for guiding user selections with the switches. Examples ofdisplayed text include, but are not limited to, “EMG”, “Limit”, Store”,and “Bat”. Examples of user selections made with the switches include,but are not limited to, setting upper and lower limits for torque valuesrepresentative of acceptable screw placements, setting upper and lowerlimits for stimulus current corresponding to acceptable screwplacements, breaches, weak bone, bone that is too hard, bone that is toosoft, and/or other conditions (LIMIT), storing preferred limit values ormeasured values of torque or stimulus current to be used as limits ordisplayed for comparison (STORE), monitoring device status such as a lowbattery indicator (BAT), and setting stimulus current magnitude andlimits (EMG).

Other examples of information 152 displayed on the status display 128 bythe microcontroller 208 include, but are not limited to, a numericaltorque value, a numerical stimulus current value, a unit applied to adisplayed value, e.g. “N-M (Newton-meter) or “mA” (milliamp), graphicalinformation such as trend lines and/or bar graphs, event identificationsuch as a record number of a screw being placed during a procedure, andso on. For example, the MODE switch 142 may be depressed until EMG isshown on the status display to prepare the IMM for outputting a stimuluscurrent for an EMG measurement. A value of stimulus current selected byoperation of the INC switch 136 and DEC switch 138 may be output undercommand of the surgeon by activation of the SELECT key.

Indicators on the front side 110 optionally include any one or more of afirst indicator 130 (HIGH limit indicator), optionally activated by theMCU 208 to alert the surgeon to a high limit being exceeded, a secondindicator 132 (ACTIVE), optionally activated by the MCU to indicateinitiation of an event such as output of a stimulus current, and a thirdindicator 134 (LOW limit indicator), optionally activated by the MCU toindicate a low limit being passed. An illuminated indicator is anexample of an activated indicator. Some operational conditionsoptionally activate more than one indicator, for example to alert a userto an error condition in the IMM, a disallowed button activation, and soon. Alternative embodiments 100 optionally include a different number ofswitches and indicators than are shown in the examples of FIG. 2 andFIG. 9 , and optionally use different switch and indicator labels thanshown in the example figures.

Examples of a motor housing 106 included with some apparatus embodiments100 are shown in FIGS. 10, 11, 12, 13, and 14 . FIGS. 10 and 11illustrate an example of a motor housing 106 with an angled hand grip222 and a chuck 230 having one or more adjustable jaws 232 for grippinga tool receptacle 122 having a drive coupling 125 implemented as a post,such as the example IMM 104 of FIG. 6 having a hexagonal post 156.Tightening the adjustable clamp jaws 232 against the flat sides of thehexagonal post 156 by rotating the chuck 230 holds the IMM firmlyagainst the motor housing with the tool receptacle 122 rotatable by themotors in the motor housing and the outer enclosure 108 stationary withrespect to the hand grip on the motor housing. The motor housing 106optionally includes an adjustable torque ring 228 for setting a maximumamount of output torque from the motors in the motor housing. The motorhousing is optionally formed with at least one, and possibly several pinreceptacles 234 configured to receive the anti-rotation pins (144, 146)on the IMM 104. Spacing pin receptacles at intervals around the chuckprovides for attachment of the IMM to the motor housing at a selectedviewing angle determined by the position of the selected pin receptacle.

FIG. 12 show an example of a hand grip 222 on an approximatelycylindrical motor housing 222. The chuck 230 in the example of FIG. 12has clamp jaws 232 similar to those on the motor housing in the examplesof FIGS. 10-11 . Any of the previously described motor—21—housingsoptionally replace the chuck with adjustable jaws with a chuck having ahexagonal drive shaft 233 shaped to engage with an IMM 104 having adrive coupling 125 implemented as a hexagonal socket 126.

Some embodiments of the IMM 104 include an internal electrical storagebattery 166 as suggested in the examples of FIG. 7 and FIG. 9 . Someembodiments of an IMM are configured to receive electrical power from anexternal source. For example, in some embodiments 100 the anti-rotationpins (144, 146) also function as electrical power connection pins andare electrically connected to the positive power terminal 148 and thecommon return terminal 150 on the IMM. The positive power terminal 148may also be referred to as the Vcc terminal 148. At least two of the pinreceptacles 234 on some embodiments of the motor housing 106 areconfigured as a positive power terminal 148 and a common return terminal150 for supplying electrical power from a battery in the motor housing(not illustrated) to circuit components in the IMM. When the battery 166in the IMM is a rechargeable battery, the motor housing 106 optionallyincludes a battery charger configured to recharge the battery in the IMMthrough the Vcc (148) and common return (150) terminals on the IMM.

FIGS. 15-17 show views of an example screw placement tool 102 includedwith some apparatus embodiments 100. The example screw placement tool inFIGS. 15-17 is configured as a screw driver for tulip head screws usedin some orthopedic procedures. A screw driver shank 236 is stronglyattached to a thumbwheel cage 238. The screw driver shank 236 optionallyterminates in a hexagonal end 254 to oppose slippage of the screw drivershank in the chuck of the motor housing. A hollow sleeve 240 is stronglyattached to the thumbwheel cage on a side opposite the screw drivershank 236. When configured for driving a tulip head screw, the hollowsleeve 240 terminates in tines 244 projecting outward from a screwengaging end 242 of the hollow sleeve. The tines 244 are positioned toengage corresponding slots or channels formed along the sides of thehead of a tulip head screw.

A shaft 246 passes through the central aperture of the hollow sleeve240. An end of the shaft 246 is attached to a thumbwheel 250 positionedinside the thumbwheel cage 238. A threaded end 248 of the shaft 246opposite the thumbwheel 250 is configured to engage with internalthreads formed in an aperture in the head of a tulip head screw.Rotating the thumbwheel in a first direction engages the threaded end248 with the threads in the tulip head screw and draws the tines 244into firm contact with the channels in the screw head, opposing rotationof the screw relative to the hollow sleeve, thumbwheel cage, and driveshank. Rotating the thumbwheel in a second direction opposite the firstdirection disengages the threaded end 248 from the screw head, allowingthe screw to separate from the tines 244. Finger grips 256 formed on thethumbwheel 250 make the thumbwheel easier to turn.

The screw placement tool 102 forms part of the continuous electricalcurrent path 258 from the IMM 104 to the tip of a screw being placed inbone. However, in some embodiments parts of the screw replacement toolmay optionally be made from materials with sufficiently high electricalresistance to impede current flow output from an IMM 104 during anevoked EMG measurement, possibly decreasing the accuracy of themeasurement. A screw placement tool therefore optionally includes anelectrical conductor 252 to establish a low-impedance current path alongthe screw driver shank 236, thumbwheel cage 238, hollow sleeve 240, andat least one of the tines 244. In some embodiments the electricalconductor 252 is affixed to the screw placement tool along the currentpath. In other embodiments the electrical conductor 252 is anintegrally-formed part of the screw placement tool. The electricalconductor 252 is preferably positioned on a tine 244 to establish goodelectrical contact with a screw head held against the tine by thethreaded end of the shaft 246.

Unless expressly stated otherwise herein, ordinary terms have theircorresponding ordinary meanings within the respective contexts of theirpresentations, and ordinary terms of art have their correspondingregular meanings.

What is claimed is:
 1. An apparatus, comprising: a screw placement tool,comprising: a screw engaging end; and an electrical conductor extendingfrom said screw engaging end to an end opposite said screw engaging end,said electrical conductor positioned to establish electrical contactwith a screw attached to said screw engaging end; and an intraoperativemonitoring module, comprising: an outer enclosure; a tool receptaclerotatably coupled to said outer enclosure, comprising: a tool socketconfigured for receiving said screw placement tool; and a contact pinextending away from an outer surface of said tool receptacle into saidtool socket; a variable-output current source having an output inelectrical communication with said electrical conductor through saidcontact pin; a microcontroller electrically connected to saidvariable-output current source; and a status display attached to saidouter enclosure, said microcontroller configured to communicate to saidstatus display a value of current output from said variable-outputcurrent source to said screw placement tool through said contact pin. 2.The apparatus of claim 1, said intraoperative monitoring module furthercomprising an anti-rotation pin extending outward from said outerenclosure adjacent said drive coupling.
 3. The apparatus of claim 1,further comprising: said contact pin positioned to contact saidelectrical conductor.
 4. The apparatus of claim 3, wherein said endopposite said screw placement end engages said tool socket with a closesliding fit.
 5. The apparatus of claim 1, said intraoperative monitoringmodule further comprising: a torque sensor attached to said toolreceptacle; and a drive coupling at an end of said tool receptacleopposite said tool socket; said microcontroller electrically connectedto said torque sensor; and said microcontroller configured to present onsaid status display a value of torque from said torque sensor.
 6. Theapparatus of claim 1, said screw placement tool further comprising: atine extending outward from said screw placement end, said tinepositioned to engage a screw head; and said electrical conductorextending over an end of said tine.